Antitheft and Alarm Systems: Description and Operation

EWS Interface EWS4

The CAS serves as an anti-theft security system and enables the start of BMW vehicles.

Function

Data transfer between CAS and DME/DDE

CAS-DME/DDE interface
A secret key, protected against manipulation, is stored in the CAS/FEM control unit and in the DME/DDE control unit.
From this point onwards, the DME/DDE and CAS/FEM control units can only be replaced with control units specifically ordered for each vehicle. An EWS adjustment in the vehicle is not possible.

IMPORTANT: Trial replacement of the DME/DDE or CAS/FEM control unit is not possible.

Key identification and starting operation
The following procedure takes place after identification of the vehicle key:
- The key sends the key data to the CAS/FEM control unit.
- The CAS/FEM control unit checks the correctness of the key data and only then enables activation of the starter by the DME/DDE.
- The DME/DDE uses a random number and the secret key to calculate a request. The DME/DDE transmits this request to CAS/FEM.
- The CAS/FEM uses the request and the secret key to calculate the response. The response is transmitted by the CAS/FEM to the DME/DDE.
- The DME/DDE itself also calculates the response that the CAS/FEM expects. The DME/DDE then checks that the response it receives from the CAS/FEM matches the response it has calculated itself.
- If the responses match, the engine start is enabled.

Fault recognition in engine control unit
The following faults are monitored in the engine control unit:
- The DME/DDE monitors both signal transmission and the signals themselves.
Engine start is inhibited if a fault is detected.